Optical film, surface light source device, and liquid crystal display device

Abstract

An optical film according to one embodiment includes at least one stack having a plurality of basic pairs, each pair being composed of a first layer and a second layer having different refractive indices and being laminated. The number of the stacks, a difference in the refractive index between the first and second layers, and the number of the basic pairs are set such that a reflectance spectrum of the at least one stack as a whole matches a target reflectance spectrum. The target reflectance spectrum includes first and second reflectance spectrum components corresponding to first and second polarized light beams in a predetermined wavelength range, wherein each of them is a spectrum having at least one reflective peak region including a spectrum region having a reflectance of 50% or more and a wavelength width of 20 to 60 nm.

The invention claimed is: 
     
       1. An optical film, comprising at least one stack having a plurality of basic pairs, each pair being composed of a first layer and a second layer having different refractive indices from each other and being laminated,
 wherein the number of the stacks, a difference in the refractive index between the first and second layers in the at least one stack, and the number of the basic pairs are set such that a reflectance spectrum of the at least one stack as a whole matches a target reflectance spectrum, 
 the target reflectance spectrum includes first and second reflectance spectrum components corresponding to first polarized light polarized in a specific direction in a wavelength range from 400 to 700 nm and second polarized light polarized in a direction orthogonal to the polarizing direction of the first polarized light, respectively, wherein each of the first and second reflectance spectrum components is a spectrum having at least one reflective peak region including a spectrum region having a reflectance of 50% or more and a wavelength width of 20 to 60 nm, 
 wherein refractive indices n i Lx, n i Ly, n i Lz of the first layer in a thickness direction and two directions orthogonal to the thickness direction are substantially equal to one another, and 
 wherein refractive indices niHx, niHy, niHz of the second layer in a thickness direction and two directions orthogonal to the thickness direction are substantially equal to one another. 
 
     
     
       2. The optical film according to  claim 1 , wherein ΔnL and ΔnH have a relation defined by an expression (1) where a difference in the refractive index within the first layer is defined as ΔnL and a difference in the refractive index within the second layer is defined as ΔnH:
 [Expression 1]
   ΔnL≦0.02 and ΔnH≦0.02  (1)
 
 
 wherein ΔnL=nL max −nL min ; ΔnH=nH max −nH max ; nL max  and aH max  are maximum refractive indices of the first and second layers, respectively, of the refractive indices of the first and second layers; nL min  and nH min  are minimum refractive indices of the first and second layers, respectively, of the refractive indices of the first and second layers. 
 
     
     
       3. The optical film according to  claim 1 , wherein Δn satisfies an expression (2) where the refractive index of the first layer is defined as nL, the refractive index of the second layer is defined as nH, and the difference in the refractive index is defined as Δn:
 [Expression  2 ]
   0.05≦Δn≦0.25  (2)
 
 
 wherein
 Δn=|nH−nL|; 
 nL=(nL max +nL min )/2; 
 nH=(nH max +nH min )/2; 
 nL max  and nH max  are the maximum refractive indices of the first and second layers, respectively, of the refractive indices of the first and second layers; and nL min and nH min are minimum refractive indices of the first and second layers, respectively, of the refractive indices of the first and second layers. 
 
 
     
     
       4. The optical film according to  claim 1 , wherein the number of the basic pairs is 25 to 50. 
     
     
       5. The optical film according to  claim 1 , wherein thicknesses of the first layer and the second layer each are 5 to 400 nm. 
     
     
       6. The optical film according to claim, wherein the number of the stacks is the number of the reflective peak regions or more. 
     
     
       7. The optical film according to  claim 1 , wherein the number of the stacks is 1 to 3. 
     
     
       8. The optical film according to  claim 1 , wherein a reflectance spectrum component of the first polarized light and that of the second polarized light in the target reflectance spectrum each have one reflective peak region in the range of 430 to 480 nm, one reflective peak region in the range of 510 to 560 nm, and one reflective peak region in the range of 600 to 660 nm. 
     
     
       9. A surface light source device, comprising:
 a light source unit, 
 a surface light-emitting element that converts light from the light source unit into planar light to emit the planar light from a light-emitting surface portion, and 
 the optical film according to  claim 1 , disposed for the surface light-emitting element on a side thereof opposite to the light-emitting surface portion. 
 
     
     
       10. A liquid crystal display device, comprising;
 a light source unit, 
 a surface light-emitting element that converts light from the light source unit into planar light to emit the planar light from a light-emitting surface portion, 
 the optical film according to  claim 1 , disposed for the surface light-emitting element on a side thereof opposite to the light-emitting surface portion, and a liquid crystal panel disposed on the light-emitting surface portion of the surface light-emitting element, the planar light entering the liquid crystal panel.